The Off-Key Bard
History

This Day in History — Helium Is Discovered in the Sun

On this day in 1868

On August 18, 1868, Janssen spotted an unknown yellow line in the Sun — humanity named an element found in a star 27 years before anyone isolated it on Earth.

The Off-Key Bard looks toward the Sun, where a thin yellow line in a spectrum is about to reveal something no one had ever encountered on Earth…

"Usually, we discover an element by digging into the Earth. This one announced itself from 93 million miles away."

On this day in 1868, French astronomer Pierre Jules César Janssen traveled to Guntur, on the eastern coast of India, to observe a total solar eclipse. The French government and national Academy of Sciences had jointly funded the expedition — more than 75,000 francs spent to position their astronomers at the precise point on Earth's surface where the Moon's shadow would fall. Totality lasted roughly six minutes, an exceptionally long window by the standards of eclipse science.

Janssen pointed his spectroscope at a solar prominence — one of those enormous loops of glowing gas that arc off the Sun's edge during eclipse — and split its light into a rainbow of thin colored lines. What he saw was mostly hydrogen, as expected. But among the lines was a bright yellow emission at a wavelength of 587.49 nanometers, positioned close to the well-known sodium doublet lines D1 and D2. Close — but not quite right. The wavelength didn't match sodium. It didn't match any element known to science. Janssen noted it carefully and moved on, uncertain what he had found.

He immediately recognized something else, however: the line was bright enough that it should be detectable without an eclipse, if he could isolate that wavelength from the overwhelming glare of the Sun. That insight led him directly to invent the spectrohelioscope — a device for photographing the Sun in a single wavelength of light — making August 18, 1868 the birthday of both a discovery and a new instrument of science.

The Independent Confirmation: Some 5,000 miles away in England, on October 20 of that same year, astronomer Joseph Norman Lockyer independently observed the same yellow line in the solar chromosphere without needing an eclipse, using his own spectroscopic technique. He and chemist Edward Frankland concluded it belonged to an entirely undiscovered element. Lockyer named it helium — from Helios, the Greek word for the Sun.

On October 20, 1868, sealed letters from Janssen and Lockyer describing their independent observations arrived at the French Academy of Sciences on the same day. To honor the coincidence, a commemorative medal was struck bearing both their faces.

Finding It Again — On Earth: For 27 years, helium existed only as a spectral signature in starlight. No one had ever held a sample of it. Then in 1882, Italian physicist Luigi Palmieri detected the same 587.49-nanometer line in volcanic gases rising from Mount Vesuvius — the first sighting of helium in earthly material, though still not isolated gas.

The breakthrough came in 1895 in London. Scottish chemist William Ramsay was hunting for argon, treating a lump of uranium-bearing mineral called cleveite with mineral acids. An unexpected gas hissed out. Ramsay ran it through his spectroscope — and there was the yellow line. He sent a sealed sample to Lockyer for confirmation. Lockyer reportedly described what he saw in the spectroscope as a "glorious yellow effulgence." The spectral signature matched exactly. Twenty-seven years after Janssen had first noticed it in an Indian eclipse, someone was finally holding a jarful of helium in a London laboratory.

The reason helium had been so difficult to find on Earth turns out to be geological. Helium is so light that any free helium at Earth's surface simply drifts upward and escapes into space. The terrestrial helium that exists is buried — produced by the slow radioactive decay of uranium and thorium deep in the crust over hundreds of millions of years, trapped inside the same geological formations that hold natural gas. Ramsay's cleveite worked because uranium inside it had been quietly generating helium since the rock first formed.

Janssen, meanwhile, never stopped chasing eclipses. In 1870, with Paris under siege during the Franco-Prussian War, he escaped the surrounded city by balloon to reach the next eclipse observation point in Algeria. Clouds obscured his view. Some discoveries require more than one attempt.

"A yellow line within the flame,
Revealed an element without a name…
Before beneath our feet 'twas found,
Its signature came from skyward ground."

History reminds us: helium's discovery proved that we don't have to touch a star to understand what it's made of — we just have to learn how to read its light. Every spectroscopic analysis of a distant galaxy today, every exoplanet atmosphere mapped by the James Webb Space Telescope, descends directly from the instrument Janssen built because he noticed a yellow line that wasn't quite sodium.

Sources

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